Two of the greatest challenges in neuro-oncology are the treatment of glioblastoma primary brain tumors and melanoma brain metastases. Both may be treated with radiation and temozolomide, but at best this merely delays the progression of these cancers. Both GBM and melanoma are marked by substantial genetic heterogeneity and by the ability to adapt to targeted therapies. This Project attempts to address both problems through targeting a novel signaling hub in cancer, diacylglycerol kinase ? (DGK?). Our prior studies of a microRNA cytotoxic to GBM cells led us to identify its knockdown of DGK? as a major driver of its cytotoxicity, indicating the potential utility of targeting this kinase. DGK? and its product phosphatidic acid had already been found important in numerous signaling pathways with oncogenic roles, further supporting the potential of DGK? as a target. We recently reported that knockdown and small-molecule inhibition of DGK? causes apoptotic cell death in GBM and melanoma lines, both in vitro and in mouse models. These studies also indicated antiangiogenic effects in vivo and the importance of mTOR and HIF-1? as mediators of DGK? effects in cancer. Since our published report, we have discovered that an abandoned medication found safe in prior clinical trials for a non-cancer indication, ritanserin, is a novel DGK? inhibitor. Importantly, recent reports suggest that DGK? inhibitors have the potential to break T cell anergy and boost cancer immunotherapies. We therefore hypothesize that ritanserin and other novel DGK? inhibitors will be highly effective against GBM and brain metastases from melanoma, both as single agents and in combination with immunotherapy. In Aim 1 of this proposal, we will test the effects of putative novel DGK? inhibitors on GBM and melanoma cell phenotype, whether these compounds affect other DGK family members, and assess possible resistance mechanisms. Aim 2 will investigate whether ritanserin and other novel DGK? inhibitors are safe and effective in GBM and melanoma mouse xenograft models. In Aim 3, we will determine in immnocompetent mice whether these DGK? inhibitors increase the local immune response and are synergistic with immunotherapy. Successful completion of the proposed studies will shed light on the biology and therapeutic targeting of DGK? in GBM and melanoma brain metastases, with the potential for rapid translation to clinical trials. This strategy may have broad applicability in cancer, acting via direct cytotoxicity to cancer cells, antiangiogenic effects, and enhancing a host of promising new immunotherapies.